The future of Spain's monarchy was cast into doubt yesterday by an opinion poll showing the country's younger voters would rather live in a republic.

The poll in El Mundo newspaper - on the 30th anniversary of the re-establishment of the monarchy after the dictator Francisco Franco died - suggested almost a quarter of Spaniards considered themselves republicans. A 50% increase in declared republicans over five years was the result of a surge in the number of 18 to 29-year-olds who preferred to scrap the monarchy, the poll showed. Declining support among young people could spell future trouble for what has previously been considered a model, modern European monarchy.

Nearly four out of 10 young voters defined themselves as republicans - slightly more than those who said they were monarchists. It was the first time in 30 years that polls had produced such a result. The result was not so worrying for King Juan Carlos - who maintains the respect of even diehard republicans after helping usher in democracy following Franco's death - as it was for his heirs.

One historian suggested the 67-year-old king should choose a suitable moment to abdicate in favour of Crown Prince Felipe to maintain his own reputation and set his son off to a good start. "Spain has experimented with republics twice in the past century and a half. There is no guarantee that such a thing might not happen again," Felipe Fernández-Armesto wrote in El Mundo.

Both the king and his son are still popular as individuals, with only 4% of Spaniards holding a poor opinion of the former and 6% disliking the latter. The decline in popularity of the monarchy comes despite self-censorship on royal matters by Spain's press, which contrasts with its eagerness to report scandals involving other European royalty.